For the quality of glasses for optical applications, in particular optical glasses, fiber-optical glasses, display glasses and/or technical glasses with stringent requirements, the absence of gas inclusions or gas bubbles and a minimum of discoloring inclusions are crucial for the unperturbed transmission of electromagnetic radiation. Furthermore, the quality of a glass is essentially influenced by its homogeneity and the absence of striations. Toxic substances or at least substances which are for concern for health or ecologically, for example silicon or antimony, should be avoided as much as possible.
In the first process step of glassmaking the starting substance, the so-called batch, is melted. After the batch has become viscously fluid owing to increase in temperature, the homogenization slowly begins, i.e. the dissolving and uniform distribution of all constituents of the melt and the elimination of striations. Initial fining likewise commences, i.e. the removal of gas bubbles from the glass melt, which is continued in further special fining steps.
Although melting and fining in the same apparatus is the most economical fining method, it is employed only for technical glasses with relatively low demands on the proportion of bubbles since the residual bubble content in this method is relatively high.
In contrast to the method described above, in continuously operated melting methods glasses with more stringent requirements, for example optical, fiber-optical or technical glasses are conventionally fined in special fining chambers or fining apparatus made of platinum or platinum alloys, in order to obtain them bubble-free. The platinum used as cladding material or bulk material is on the one hand very cost-intensive, and on the other hand apparatus made of platinum or platinum alloys have the disadvantage that small amounts of Pt or other alloy constituents are released into the melt owing to the corrosiveness and in part reactivity of the glass melts. Depending on the redox state of the glass, these alloy constituents will be present in ionic form, for example as the Pt4+ ion or Rh4+ ions, or as colloidal particles finely distributed in elementary form in the glass end product. Depending on the concentration and/or particle size in the glass end product, this introduction of ionic or elementary metal into the glass melt may lead to undesired discoloration and reduced transmission of electromagnetic radiation, not only in the visible range.
Another possible way to improve the fining of the glass and shorten the fining time consists in using high fining temperatures. Increasing the temperature during the fining will inter alia reduce the viscosity of the glass melt and thereby increase the rate of ascent of the bubbles present in the glass melt.
At elevated fining temperatures, particularly above 1550° C. or when fining corrosive glasses, increased attack of the apparatus wall by the glass leads to increased introduction of material into the glass melt and therefore into the glass end product. Furthermore, there is also a detrimental restriction in respect of high fining temperatures since apparatus made of platinum can be used only up to a temperature of at most 1600° C. and apparatus made of PtRh10, an alloy which consists of 90 wt. % platinum and 10 wt. % rhodium, can be used only up to at most 1700° C. The high input of material leads to a strong yellow coloration of the glass. Apparatus made of PtRh20, an alloy which consists of 80 wt. % platinum and 20 wt. % rhodium, can be used up at most 1800° C., which likewise leads to a strong yellow coloration of the glass. Apparatus made of ZrO2-stabilized platinum can be used only up to 1650° C.
A further avenue for optimizing the fining resides in the use of chemical fining agents. The principle of this method consists in adding constituents to the batch, which decompose in the melt at high temperatures to evolve or release gas, generally oxygen. The gas released by the fining agents absorbs the gases contained in the melt, creating bubbles that grow as the fining time increases, which more rapidly ascend to the surface of the melt and therefore leave the melt.
Inter alia, the choice of the fining agents depends on the temperature of the glass melt during the fining since the evolution of gas or decomposition of the various fining agents take place at different temperatures. For example the fining agent arsenic pentoxide, As2O5, already decomposes by cleaving oxygen at a temperature above 1250° C. into arsenic oxide, As2O3, which remains in the glass melt and is therefore contained in the glass end product. Conversely a so-called high-temperature fining agent, for example SnO2, is only to be used at a temperature above 1500° C. SnO2 decomposes above temperatures of 1500° C. into SnO and ½ O2. The oxides formed remain for the most part in the melt and are detectable in the glass end product. Arsenic present in the glass end product is particularly disadvantageous when ecologically and health-safe glasses are desired. There is therefore a pressing need for possible ways of fining at high temperatures, in order to be able to carry out the fining more efficiently so as to obtain a product with the least possible bubbles.
Document U.S. Pat. No. 6,632,086 B1 describes a glass melting crucible which makes it possible to carry out fining of the glass at temperatures of up to 2350° C. Owing to better, solubility of gases in the starting material, operating the device at such high temperatures leads to less bubble defects, less discolorations and minimal formation of striations. The device comprises a body made of ceramic refractory material, which is coated on the side facing the glass with a 0.25 mm to 1.27 mm thick nonreactive boundary layer of rhenium, osmium, iridium or a mixture thereof.
As described above, bubbles present in the glass melt not only have their origin in the starting products, rather they may also be due to thermal dissociation of water contained in the melt.
The documents described below deal with this phenomenon of bubble formation.
Document WO 02/44115 A2 describes a coated metal part for glassmaking, which has a layer impermeable to H2 or H2 and O2 on the other side from the glass melt. As a function of the temperature, the water present in the glass melt dissociates into hydrogen and oxygen. While hydrogen can diffuse through the wall material, this is not possible for the oxygen being formed owing to its size. The effect of the hydrogen diffusion is that an equilibrium state between hydrogen and oxygen is no longer achieved in the melt, such that water would form again when cooling the melt. If the O2 concentration in the melt finally exceeds the solubility limit at an O2 partial pressure of about 1 bar, then bubbles containing O2 will be formed. The bubbles grow even more owing to the diffusion of SO2, N2, CO2, and other gases physically dissolved in the glass, into them during cooling and they are detectable in the finished product. This greatly compromises the quality of the glass products being produced.
It is known from Patent U.S. Pat. No. 5,785,726 that containers made of platinum or platinum alloys can be protected against the creation of electrochemically formed O2 bubbles by flushing with an atmosphere containing hydrogen or steam.
WO 98/18731 describes the prevention of bubble formation in the contact zones between a glass melt and platinum or molybdenum by applying a hydrogen atmosphere on the other side from the glass melt. The H2 partial pressure on the outside prevents the diffusion of H2 from the melt through the platinum wall.
However, the methods mentioned last require elaborate monitoring and control. On the one hand they show a large susceptibility to error and on the other hand, owing to the hazardous nature of hydrogen, they entail a high risk per se. Faults with the control and regulation lead to expensive production down times.